1. Field of the Invention
This invention relates to chemical-mechanical polishing ("CMP") and, more particularly, to an apparatus for conditioning a polishing pad during CMP.
2. Description of Related Art
The concept of applying chemical and mechanical abrasion to a substrate is generally well known in the industry as CMP. A typical CMP process involves placing a substrate or, according to one specific application, semiconductor wafer face-down on a polishing pad which is fixedly attached to a rotatable table or platen. Elevationally extending portions of the downward-directed wafer surface contact with the rotating pad. A fluid-based chemical, often referred to as a "slurry" is deposited upon the pad possibly through a nozzle, whose distal opening is placed proximate the pad laterally offset from the wafer. The slurry extends at the interface between the pad and the wafer surface to initiate the polishing process by chemically reacting with the surface material being polished. The polishing pad is facilitated by the rotational movement of the pad relative to the wafer (or vice versa) to remove material catalyzed by the slurry.
The slurry can be made of numerous chemical species depending on the material being removed from the wafer surface. For example, the slurry can comprise silica, alumina or ceria particles entrained within, e.g., a potassium-based solvent. The solvent may include, for example, potassium hydroxide, potassium ferracyanide, potassium acetate or potassium fluoride diluted with deionized water. The amount of particulate in the solvent can be selected and sold under various trade names, a suitable source being Semi-Spurse.RTM. or Cab-O-Sperse.RTM., manufactured by Cabot, Inc. Merely as an example, a preferred slurry composition for CMP of a tungsten film is a solution comprising approximately 0.1 molar potassium ferracyanide, approximately 5% by weight silica, with trace amounts of potassium acetate diluted with deionized water. A small amount of concentrated acetic acid is further included to adjust the pH of the tungsten slurry to a range between, e.g., 3.0 to 4.0. The above-referenced composition proves beneficial in removing tungsten from interlevel silicon dioxide layers.
The polishing pad can be made of various substances which can be both resilient and, to a lesser extend, conformal. The weight, density and hardness of the pad will vary depending on the material being removed from the wafer surface. A popular polishing pad comprises polyurethane which, in most instances, does not include an overlying fabric material. A somewhat hard polishing pad can be obtained as an IC-60 pad manufactured by Rodel Corporation, for example. A relatively soft pad can be obtained as a Polytech Supreme pad, also manufactured by Rodel Corp.
Use of both chemical and mechanical abrasion in a CMP environment is popular in most modern semiconductor wafer fabrication processes. For example, CMP is used to planarize tungsten-based interconnect plugs commensurate with the upper surface of the interlevel dielectric. The remaining tungsten is bounded exclusively within the interconnect regions which extend between levels of interconnect. As another example, CMP may be used to planarize fill dielectric placed in shallow trenches, the planarized fill dielectric is thereby used as a field dielectric. Accordingly, CMP is principally popular as a planarization tool.
CMP may be used several times throughout a modern semiconductor wafer fabrication process. Unfortunately, each time CMP is used, various items are "consumed". These items include the slurry and, more importantly, wear upon the polishing pad. The polishing pad is relatively expensive and after several CMP runs, must either be replaced or "conditioned".
Pad conditioning is typically performed by mechanically abraiding the pad surface in order to renew that surface. Most conditioning devices can renew the surface during wafer polish. Current conditioning processes involve placing a conditioning head with an abrasive surface over the polish pad in a region laterally displaced from the semiconductor wafer. As the polishing pad rotates, the conditioning head displaces the abrasive surface upon and within the polishing pad in the track through which the wafer extends. The abrasive surface contacting the polishing pad renews the surface by removing depleted slurry particles or polishing by-product embedded in the pores of the polishing pad. Opening the pores allow new slurry to enter the pores to enhance polishing therein. Additionally, the open pores provide more surface area for polishing.
If the pores remain blocked over a substantial period of time, a condition known as "glazing" occurs. Glazing is the result of enough particle build-up on the polishing pad surface that the wafer surface begins to hydroplane over the surface of the pad. Hydroplaning eventually result in substantially lower removal rates in the glazed areas.
An example in which a polishing pad is conditioned concurrent with wafer polishing is shown in FIG. 1. FIG. 1 provides a perspective view of a polishing pad 10 mounted on a rotatable platen 12. Platen 12 rotates about a central axis 14 along the direction shown by arrow 16. Platen 12, including pad 10, can be directed upward against wafer 18 (or vice versa). Wafer 18 is secured in a rotatable position about axis 20 by an arm 22. Wafer 18 is mounted such that the frontside surface extends against pad 10, the frontside surface embodying numerous topological features used in producing an integrated circuit. Wafer 18 rotates about axis 20 along arrow 24 within a plane parallel to the plane formed by the polishing surface of pad 10.
Wafer 18 occupies a portion of the polishing surface, denoted as a circular track 26 defined by the rotational movement of pad 10. Track 26 is conditioned during wafer polish by a conditioning head 28. Conditioning head 28 is mounted on a movable arm 30 which can swing in position along track 26 commensurate with arm 22. Arm 30 presses an abrasive surface of conditioning head 28 against the polishing surface of pad 10 predominantly within track 26 as pad 10 rotates about axis 14. During this process, protrusions on the abrasive, downward-facing surface of head 28 extend to the surface of polishing pad 10. This causes particles embedded in the pores of pad 10 to be removed from the pad and flushed with the slurry across the pad surface. As the slurry is introduced, the removed particles are rinsed over the edges of the polishing pad into a drain (not shown). Removing the particles from the polishing pad enables the depleted pad surface to be recharged with new slurry. FIG. 1 illustrates conditioning concurrent with wafer polishing. However, it is recognized that conventional conditioning can occur either before or after wafer polishing.
FIG. 2 illustrates a cross-sectional view of the CMP and conditioning process illustrated in FIG. 1. More specifically, FIG. 2 illustrates the abrasive surface 32 formed at the lower surface of conditioning head 28. Abrasive surface 32 extends as a plurality of protrusions interspersed with recesses. The protrusions and recesses can be spaced close together or farther apart depending on the porosity of pad 10. Surface 32 preferably contacts pad 10 surface commensurate with wafer 18. More particularly, abrasive surface 32 extends below the upper surface of slurry film 34 to dislodge depleted slurry particles and/or wafer polish by-product from pores of pad 10.
Concurrent pad conditioning with wafer polishing enhances the throughput of the CMP process. Little if any downtime is therefore associated with pad conditioning. Unfortunately, however, conditioning head 28 occupies area upon pad 10 which might better be used by additional semiconductor wafers. In other words, it would be desirable to further enhance CMP throughput by configuring multiple wafers across the polishing surface. Introduction of multiple wafers, however, would forgo the space needed for conventional conditioning. Yet further, it would be desirable to eliminate the conditioning head and the relatively costly mechanism for moving and aligning the head to the polishing track 26 upon the polishing surface. The additional mechanical components and abrasive surfaces afforded by conventional conditioning heads should be eliminated if cost minimization and throughput is to be enhanced.